The preparation of diesters of oxalic acids (oxalates) is of particular interest to the chemical industry owing to the varied uses of these compounds. Not only may these diesters serve to act as the starting materials for such important compounds as oxalic acid, oxamide or ethylene glycol, but they may also find extensive use as intermediates for dyes, pharmaceuticals and the like.
Prior to the instant invention there have been proposed conventional liquid phase processes for the formation of these diesters which employ various catalyst systems, co-catalysts, reaction accelerators and the like. However, these conventional processes have suffered from the serious effects arising from conventional liquid phase processes.
A particularly interesting process for preparing diesters of oxalic acid in the liquid phase is disclosed in U.S. Pat. No. 4,138,587. U.S. Pat. No. 4,138,587 employs nitric acid or a nitrogen oxide as the accelerator in the presence of a solid platinum group metal or salt thereof, alcohol, molecular oxygen and carbon monoxide. The process is a liquid phase process and suffers from significant practical defects related to liquid phase processes, such as catalyst losses, large by-product formation, low efficiencies to product, to name a few.
U.S. Pat. No. 4,229,591 discloses, in part, a vapor phase process. The process disclosed therein involves contacting an ester of nitrous acid with carbon monoxide in the gaseous phase under ordinary pressure in the presence of a solid catalyst containing metallic palladium or a salt thereof at a temperature of 50.degree. to 200.degree. C., in which the ester of nitrous acid is an ester of nitrous acid with an alcohol having 1 to 8 carbon atoms such as saturated monohydric aliphatic alcohols and alicyclic alcohols. Although this process provides advantages over the corresponding liquid phase process, the vapor phase process does not provide a completely vapor phase process in which the alkyl nitrite is also prepared in the vapor state in a separate step. An exception described in this patent involves nitrite formation in situ in the oxalate reactor concurrent with oxalate formation. Such in situ formation adds a potential hazardous procedure and introduces water into the reactor in amounts which affect the production of significant quantities of undesirable by-products.
Japanese Application No. 53-8268/78 describes the preparation of nitrite esters by conventional liquid phase processes as part of the continuous production of oxalic acid diester using nitrite ester as a starting material by the vapor phase oxalate process of U.S. Pat. No. 4,229,591. The nitrous acid ester is formed by employing a gas-liquid contacting apparatus to contact a gaseous nitrogen oxide mixture with liquid alcohol at a temperature lower than the boiling point of the alcohol.
There is foreseen herein an advantage of having a multi-step wholly vapor phase process where the presence of undesirable amounts of water in the oxalate reaction can be avoided. For example, effecting the process in two vapor phase reaction zones both greatly improves the conversion of reactants to desired product (the diester of oxalic acid) and minimizes by-product formation. Other advantages are achieved, such as enhancement of catalyst life, due to minimization of deleterious by-products which can attack the catalyst either to dissolve a portion of it or to form stable and unreactive species.
In order to have a "true" vapor state process the entire process must be effected in the vapor state and, further, if the process is to be optimized, the nitrite formation should be in a reaction zone apart from the oxalate forming reaction zone. In the present process, because the ester of nitrous acid is prepared in the vapor state in a nitrous ester forming reaction zone the overall process minimizes by-product formation, enhances catalyst life, provides higher conversions to and yields of the diester of oxalic acid, and the like. By correlating the process steps so as to have an integrated vapor state process, i.e., have the formation of both the ester of nitrous acid and the diester of oxalic acid in the vapor state, in nitrous acid ester forming and oxalate forming reaction zones, respectively, an optimized process is obtained.
The classical method for preparing nitrite esters involves the liquid phase reaction of sodium nitrite and sulfuric acid with a desired alcohol. The reaction is normally carried out at ice temperatures, due to the extremely exothermic nature of the reaction, to form nitrite ester as follows: EQU 2NaNO.sub.2 +H.sub.2 SO.sub.4 +2ROH.fwdarw.2RONO+Na.sub.2 SO.sub.4 +2H.sub.2 O
The nitrite ester formed is essentially insoluble in water (less than about 1 percent in water or water in the nitrite ester) so that the nitrite ester is separated from the reaction products by liquid separation.
The production of nitrite esters in the liquid phase is disclosed in U.S. Pat. No. 2,166,698 wherein nitrite esters are produced by reacting an appreciably water soluble saturated open-chain aliphatic compound containing a plurality of esterifiable carbinol groups with nitrous acid in an aqueous medium and removing a nitrite ester from the reaction system substantially as soon as it is formed therein. The nitrite esters formed therein react rapidly with alcohols, e.g., ethyl alcohol, to form alkyl nitrites, e.g., ethyl nitrite.
In U.S. Pat. No. 2,739,166 alkyl nitrites are produced in a liquid phase process by bubbling nitrogen dioxide gas into a cooled liquid monohydric aliphatic alcohol.
In British Patent Specification No. 586,022 a liquid phase process is disclosed for the preparation of nitric acid esters which comprises reacting an alcohol with dinitrogen tetroxide in the liquid phase.
In German Patentschrift No. 1,156,775 a liquid phase process is disclosed for preparing esters of nitrous acid by employing alcohol in molar excess over dinitrogen trioxide at temperatures below the boiling point of the alcohol and simultaneously distilling off the ester formed. In addition, the reference acknowledges that the vapor phase decomposition of alcohols with nitrogen dioxide-nitrogen monoxide mixtures at temperatures between 100.degree. and 420.degree. C. is known.
The aforementioned processes are to be distinguished from a vapor phase process that may be employed in the process of this invention in that in the liquid phase processes the separation of the nitrite ester product is difficult and oxidation of alcohol in the liquid phase during the manufacture or separation can occur to form unwanted by-products. In addition, the separation of the highly flammable and toxic nitrite ester from the liquid phase can prove to be a major safety and health problem. The resulting ester of nitrous acid can be obtained only after separation and purification.
A vapor phase process for the preparation of ester of nitrous acid is disclosed in U.S. Pat. No. 2,831,882. The patent's process comprises reacting a vaporized alcohol with from 0.4 to 0.6 mole of nitrogen dioxide and 0.4 to 2.0 moles of nitric oxide per mole of alcohol in the presence of from 2 to 25 moles of diluent selected from the group consisting of water, nitrogen, and carbon dioxide at a temperature between 100.degree. C. and 420.degree. C. with a contact time of 1-10 seconds.